/*
 * Written by Doug Lea with assistance from members of JCP JSR-166 Expert Group and released to the
 * public domain, as explained at http://creativecommons.org/publicdomain/zero/1.0/
 */
package alluxio.concurrent.jsr;

import java.io.Serializable;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.lang.reflect.Constructor;
import java.util.Collection;
import java.util.List;
import java.util.Objects;
import java.util.RandomAccess;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Abstract base class for tasks that run within a {@link ForkJoinPool}. A {@code ForkJoinTask} is a
 * thread-like entity that is much lighter weight than a normal thread. Huge numbers of tasks and
 * subtasks may be hosted by a small number of actual threads in a ForkJoinPool, at the price of
 * some usage limitations.
 *
 * <p>
 * A "main" {@code ForkJoinTask} begins execution when it is explicitly submitted to a
 * {@link ForkJoinPool}, or, if not already engaged in a ForkJoin computation, commenced in the
 * {@link ForkJoinPool#commonPool()} via {@link #fork}, {@link #invoke}, or related methods. Once
 * started, it will usually in turn start other subtasks. As indicated by the name of this class,
 * many programs using {@code ForkJoinTask} employ only methods {@link #fork} and {@link #join}, or
 * derivatives such as {@link #invokeAll(ForkJoinTask...) invokeAll}. However, this class also
 * provides a number of other methods that can come into play in advanced usages, as well as
 * extension mechanics that allow support of new forms of fork/join processing.
 *
 * <p>
 * A {@code ForkJoinTask} is a lightweight form of {@link Future}. The efficiency of
 * {@code ForkJoinTask}s stems from a set of restrictions (that are only partially statically
 * enforceable) reflecting their main use as computational tasks calculating pure functions or
 * operating on purely isolated objects. The primary coordination mechanisms are {@link #fork}, that
 * arranges asynchronous execution, and {@link #join}, that doesn't proceed until the task's result
 * has been computed. Computations should ideally avoid {@code synchronized} methods or blocks, and
 * should minimize other blocking synchronization apart from joining other tasks or using
 * synchronizers such as Phasers that are advertised to cooperate with fork/join scheduling.
 * Subdividable tasks should also not perform blocking I/O, and should ideally access variables that
 * are completely independent of those accessed by other running tasks. These guidelines are loosely
 * enforced by not permitting checked exceptions such as {@code IOExceptions} to be thrown. However,
 * computations may still encounter unchecked exceptions, that are rethrown to callers attempting to
 * join them. These exceptions may additionally include {@link RejectedExecutionException} stemming
 * from internal resource exhaustion, such as failure to allocate internal task queues. Rethrown
 * exceptions behave in the same way as regular exceptions, but, when possible, contain stack traces
 * (as displayed for example using {@code ex.printStackTrace()}) of both the thread that initiated
 * the computation as well as the thread actually encountering the exception; minimally only the
 * latter.
 *
 * <p>
 * It is possible to define and use ForkJoinTasks that may block, but doing so requires three
 * further considerations: (1) Completion of few if any <em>other</em> tasks should be dependent on
 * a task that blocks on external synchronization or I/O. Event-style async tasks that are never
 * joined (for example, those subclassing {@link CountedCompleter}) often fall into this category.
 * (2) To minimize resource impact, tasks should be small; ideally performing only the (possibly)
 * blocking action. (3) Unless the {@link ForkJoinPool.ManagedBlocker} API is used, or the number of
 * possibly blocked tasks is known to be less than the pool's {@link ForkJoinPool#getParallelism}
 * level, the pool cannot guarantee that enough threads will be available to ensure progress or good
 * performance.
 *
 * <p>
 * The primary method for awaiting completion and extracting results of a task is {@link #join}, but
 * there are several variants: The {@link Future#get} methods support interruptible and/or timed
 * waits for completion and report results using {@code Future} conventions. Method {@link #invoke}
 * is semantically equivalent to {@code fork(); join()} but always attempts to begin execution in
 * the current thread. The "<em>quiet</em>" forms of these methods do not extract results or report
 * exceptions. These may be useful when a set of tasks are being executed, and you need to delay
 * processing of results or exceptions until all complete. Method {@code invokeAll} (available in
 * multiple versions) performs the most common form of parallel invocation: forking a set of tasks
 * and joining them all.
 *
 * <p>
 * In the most typical usages, a fork-join pair act like a call (fork) and return (join) from a
 * parallel recursive function. As is the case with other forms of recursive calls, returns (joins)
 * should be performed innermost-first. For example, {@code a.fork();
 * b.fork(); b.join(); a.join();} is likely to be substantially more efficient than joining
 * {@code a} before {@code b}.
 *
 * <p>
 * The execution status of tasks may be queried at several levels of detail: {@link #isDone} is true
 * if a task completed in any way (including the case where a task was cancelled without executing);
 * {@link #isCompletedNormally} is true if a task completed without cancellation or encountering an
 * exception; {@link #isCancelled} is true if the task was cancelled (in which case
 * {@link #getException} returns a {@link java.util.concurrent.CancellationException}); and
 * {@link #isCompletedAbnormally} is true if a task was either cancelled or encountered an
 * exception, in which case {@link #getException} will return either the encountered exception or
 * {@link java.util.concurrent.CancellationException}.
 *
 * <p>
 * The ForkJoinTask class is not usually directly subclassed. Instead, you subclass one of the
 * abstract classes that support a particular style of fork/join processing, typically
 * {@link java.util.concurrent.RecursiveAction} for most computations that do not return results,
 * {@link java.util.concurrent.RecursiveTask} for those that do, and
 * {@link java9.util.concurrent.CountedCompleter} for those in which completed actions trigger other
 * actions. Normally, a concrete ForkJoinTask subclass declares fields comprising its parameters,
 * established in a constructor, and then defines a {@code compute} method that somehow uses the
 * control methods supplied by this base class.
 *
 * <p>
 * Method {@link #join} and its variants are appropriate for use only when completion dependencies
 * are acyclic; that is, the parallel computation can be described as a directed acyclic graph
 * (DAG). Otherwise, executions may encounter a form of deadlock as tasks cyclically wait for each
 * other. However, this framework supports other methods and techniques (for example the use of
 * {@link java.util.concurrent.Phaser}, {@link #helpQuiesce}, and {@link #complete}) that may be of
 * use in constructing custom subclasses for problems that are not statically structured as DAGs. To
 * support such usages, a ForkJoinTask may be atomically <em>tagged</em> with a {@code short} value
 * using {@link #setForkJoinTaskTag} or {@link #compareAndSetForkJoinTaskTag} and checked using
 * {@link #getForkJoinTaskTag}. The ForkJoinTask implementation does not use these {@code protected}
 * methods or tags for any purpose, but they may be of use in the construction of specialized
 * subclasses. For example, parallel graph traversals can use the supplied methods to avoid
 * revisiting nodes/tasks that have already been processed. (Method names for tagging are bulky in
 * part to encourage definition of methods that reflect their usage patterns.)
 *
 * <p>
 * Most base support methods are {@code final}, to prevent overriding of implementations that are
 * intrinsically tied to the underlying lightweight task scheduling framework. Developers creating
 * new basic styles of fork/join processing should minimally implement {@code protected} methods
 * {@link #exec}, {@link #setRawResult}, and {@link #getRawResult}, while also introducing an
 * abstract computational method that can be implemented in its subclasses, possibly relying on
 * other {@code protected} methods provided by this class.
 *
 * <p>
 * ForkJoinTasks should perform relatively small amounts of computation. Large tasks should be split
 * into smaller subtasks, usually via recursive decomposition. As a very rough rule of thumb, a task
 * should perform more than 100 and less than 10000 basic computational steps, and should avoid
 * indefinite looping. If tasks are too big, then parallelism cannot improve throughput. If too
 * small, then memory and internal task maintenance overhead may overwhelm processing.
 *
 * <p>
 * This class provides {@code adapt} methods for {@link Runnable} and {@link Callable}, that may be
 * of use when mixing execution of {@code ForkJoinTasks} with other kinds of tasks. When all tasks
 * are of this form, consider using a pool constructed in <em>asyncMode</em>.
 *
 * <p>
 * ForkJoinTasks are {@code Serializable}, which enables them to be used in extensions such as
 * remote execution frameworks. It is sensible to serialize tasks only before or after, but not
 * during, execution. Serialization is not relied on during execution itself.
 *
 * @since 1.7
 * @author Doug Lea
 */
public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
  // CVS rev. 1.117

  /*
   * See the internal documentation of class ForkJoinPool for a general implementation overview.
   * ForkJoinTasks are mainly responsible for maintaining their "status" field amidst relays to
   * methods in ForkJoinWorkerThread and ForkJoinPool.
   *
   * The methods of this class are more-or-less layered into (1) basic status maintenance (2)
   * execution and awaiting completion (3) user-level methods that additionally report results. This
   * is sometimes hard to see because this file orders exported methods in a way that flows well in
   * javadocs.
   */

  /**
   * The status field holds run control status bits packed into a single int to minimize footprint
   * and to ensure atomicity (via CAS). Status is initially zero, and takes on nonnegative values
   * until completed, upon which status (anded with DONE_MASK) holds value NORMAL, CANCELLED, or
   * EXCEPTIONAL. Tasks undergoing blocking waits by other threads have the SIGNAL bit set.
   * Completion of a stolen task with SIGNAL set awakens any waiters via notifyAll. Even though
   * suboptimal for some purposes, we use basic builtin wait/notify to take advantage of "monitor
   * inflation" in JVMs that we would otherwise need to emulate to avoid adding further per-task
   * bookkeeping overhead. We want these monitors to be "fat", i.e., not use biasing or thin-lock
   * techniques, so use some odd coding idioms that tend to avoid them, mainly by arranging that
   * every synchronized block performs a wait, notifyAll or both.
   *
   * These control bits occupy only (some of) the upper half (16 bits) of status field. The lower
   * bits are used for user-defined tags.
   */

  static final int DONE_MASK = 0xf0000000; // mask out non-completion bits
  static final int NORMAL = 0xf0000000; // must be negative
  static final int CANCELLED = 0xc0000000; // must be < NORMAL
  static final int EXCEPTIONAL = 0x80000000; // must be < CANCELLED
  static final int SIGNAL = 0x00010000; // must be >= 1 << 16
  static final int SMASK = 0x0000ffff; // short bits for tags
  /**
   * Hash table of exceptions thrown by tasks, to enable reporting by callers. Because exceptions
   * are rare, we don't directly keep them with task objects, but instead use a weak ref table. Note
   * that cancellation exceptions don't appear in the table, but are instead recorded as status
   * values.
   *
   * The exception table has a fixed capacity.
   */
  private static final ExceptionNode[] exceptionTable = new ExceptionNode[32];
  /** Lock protecting access to exceptionTable. */
  private static final ReentrantLock exceptionTableLock = new ReentrantLock();
  /** Reference queue of stale exceptionally completed tasks. */
  private static final ReferenceQueue<ForkJoinTask<?>> exceptionTableRefQueue =
      new ReferenceQueue<>();
  private static final long serialVersionUID = -7721805057305804111L;
  // Unsafe mechanics
  private static final sun.misc.Unsafe U = UnsafeAccess.unsafe;
  private static final long STATUS;

  static {
    try {
      STATUS = U.objectFieldOffset(ForkJoinTask.class.getDeclaredField("status"));
    } catch (Exception e) {
      throw new ExceptionInInitializerError(e);
    }
  }

  /** The run status of this task */
  volatile int status; // accessed directly by pool and workers

  // Exception table support

  /**
   * Cancels, ignoring any exceptions thrown by cancel. Used during worker and pool shutdown. Cancel
   * is spec'ed not to throw any exceptions, but if it does anyway, we have no recourse during
   * shutdown, so guard against this case.
   */
  static final void cancelIgnoringExceptions(ForkJoinTask<?> t) {
    if (t != null && t.status >= 0) {
      try {
        t.cancel(false);
      } catch (Throwable ignore) {
      }
    }
  }

  /**
   * Polls stale refs and removes them. Call only while holding lock.
   */
  private static void expungeStaleExceptions() {
    for (Object x; (x = exceptionTableRefQueue.poll()) != null;) {
      if (x instanceof ExceptionNode) {
        ExceptionNode[] t = exceptionTable;
        int i = ((ExceptionNode) x).hashCode & (t.length - 1);
        ExceptionNode e = t[i];
        ExceptionNode pred = null;
        while (e != null) {
          ExceptionNode next = e.next;
          if (e == x) {
            if (pred == null)
              t[i] = next;
            else
              pred.next = next;
            break;
          }
          pred = e;
          e = next;
        }
      }
    }
  }

  /**
   * If lock is available, polls stale refs and removes them. Called from ForkJoinPool when pools
   * become quiescent.
   */
  static final void helpExpungeStaleExceptions() {
    final ReentrantLock lock = exceptionTableLock;
    if (lock.tryLock()) {
      try {
        expungeStaleExceptions();
      } finally {
        lock.unlock();
      }
    }
  }

  /**
   * A version of "sneaky throw" to relay exceptions.
   */
  static void rethrow(Throwable ex) {
    ForkJoinTask.<RuntimeException>uncheckedThrow(ex);
  }

  /**
   * The sneaky part of sneaky throw, relying on generics limitations to evade compiler complaints
   * about rethrowing unchecked exceptions.
   */
  @SuppressWarnings("unchecked")
  static <T extends Throwable> void uncheckedThrow(Throwable t) throws T {
    if (t != null)
      throw (T) t; // rely on vacuous cast
    else
      throw new Error("Unknown Exception");
  }

  /**
   * Forks the given tasks, returning when {@code isDone} holds for each task or an (unchecked)
   * exception is encountered, in which case the exception is rethrown. If more than one task
   * encounters an exception, then this method throws any one of these exceptions. If any task
   * encounters an exception, the other may be cancelled. However, the execution status of
   * individual tasks is not guaranteed upon exceptional return. The status of each task may be
   * obtained using {@link #getException()} and related methods to check if they have been
   * cancelled, completed normally or exceptionally, or left unprocessed.
   *
   * @param t1 the first task
   * @param t2 the second task
   * @throws NullPointerException if any task is null
   */
  public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {
    int s1, s2;
    t2.fork();
    if ((s1 = t1.doInvoke() & DONE_MASK) != NORMAL)
      t1.reportException(s1);
    if ((s2 = t2.doJoin() & DONE_MASK) != NORMAL)
      t2.reportException(s2);
  }

  /**
   * Forks the given tasks, returning when {@code isDone} holds for each task or an (unchecked)
   * exception is encountered, in which case the exception is rethrown. If more than one task
   * encounters an exception, then this method throws any one of these exceptions. If any task
   * encounters an exception, others may be cancelled. However, the execution status of individual
   * tasks is not guaranteed upon exceptional return. The status of each task may be obtained using
   * {@link #getException()} and related methods to check if they have been cancelled, completed
   * normally or exceptionally, or left unprocessed.
   *
   * @param tasks the tasks
   * @throws NullPointerException if any task is null
   */
  public static void invokeAll(ForkJoinTask<?>... tasks) {
    Throwable ex = null;
    int last = tasks.length - 1;
    for (int i = last; i >= 0; --i) {
      ForkJoinTask<?> t = tasks[i];
      if (t == null) {
        if (ex == null)
          ex = new NullPointerException();
      } else if (i != 0)
        t.fork();
      else if (t.doInvoke() < NORMAL && ex == null)
        ex = t.getException();
    }
    for (int i = 1; i <= last; ++i) {
      ForkJoinTask<?> t = tasks[i];
      if (t != null) {
        if (ex != null)
          t.cancel(false);
        else if (t.doJoin() < NORMAL)
          ex = t.getException();
      }
    }
    if (ex != null)
      rethrow(ex);
  }

  /**
   * Forks all tasks in the specified collection, returning when {@code isDone} holds for each task
   * or an (unchecked) exception is encountered, in which case the exception is rethrown. If more
   * than one task encounters an exception, then this method throws any one of these exceptions. If
   * any task encounters an exception, others may be cancelled. However, the execution status of
   * individual tasks is not guaranteed upon exceptional return. The status of each task may be
   * obtained using {@link #getException()} and related methods to check if they have been
   * cancelled, completed normally or exceptionally, or left unprocessed.
   *
   * @param tasks the collection of tasks
   * @param <T> the type of the values returned from the tasks
   * @return the tasks argument, to simplify usage
   * @throws NullPointerException if tasks or any element are null
   */
  public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) {
    if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) {
      invokeAll(tasks.toArray(new ForkJoinTask<?>[0]));
      return tasks;
    }
    List<? extends ForkJoinTask<?>> ts = (List<? extends ForkJoinTask<?>>) tasks;
    Throwable ex = null;
    int last = ts.size() - 1;
    for (int i = last; i >= 0; --i) {
      ForkJoinTask<?> t = ts.get(i);
      if (t == null) {
        if (ex == null)
          ex = new NullPointerException();
      } else if (i != 0)
        t.fork();
      else if (t.doInvoke() < NORMAL && ex == null)
        ex = t.getException();
    }
    for (int i = 1; i <= last; ++i) {
      ForkJoinTask<?> t = ts.get(i);
      if (t != null) {
        if (ex != null)
          t.cancel(false);
        else if (t.doJoin() < NORMAL)
          ex = t.getException();
      }
    }
    if (ex != null)
      rethrow(ex);
    return tasks;
  }

  /**
   * Possibly executes tasks until the pool hosting the current task
   * {@linkplain ForkJoinPool#isQuiescent is quiescent}. This method may be of use in designs in
   * which many tasks are forked, but none are explicitly joined, instead executing them until all
   * are processed.
   */
  public static void helpQuiesce() {
    Thread t;
    if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
      ForkJoinWorkerThread wt = (ForkJoinWorkerThread) t;
      wt.pool.helpQuiescePool(wt.workQueue);
    } else
      ForkJoinPool.quiesceCommonPool();
  }

  /**
   * Returns the pool hosting the current thread, or {@code null} if the current thread is executing
   * outside of any ForkJoinPool.
   *
   * <p>
   * This method returns {@code null} if and only if {@link #inForkJoinPool} returns {@code false}.
   *
   * @return the pool, or {@code null} if none
   */
  public static ForkJoinPool getPool() {
    Thread t = Thread.currentThread();
    return (t instanceof ForkJoinWorkerThread) ? ((ForkJoinWorkerThread) t).pool : null;
  }

  /**
   * Returns {@code true} if the current thread is a {@link ForkJoinWorkerThread} executing as a
   * ForkJoinPool computation.
   *
   * @return {@code true} if the current thread is a {@link ForkJoinWorkerThread} executing as a
   *         ForkJoinPool computation, or {@code false} otherwise
   */
  public static boolean inForkJoinPool() {
    return Thread.currentThread() instanceof ForkJoinWorkerThread;
  }

  /**
   * Returns an estimate of the number of tasks that have been forked by the current worker thread
   * but not yet executed. This value may be useful for heuristic decisions about whether to fork
   * other tasks.
   *
   * @return the number of tasks
   */
  public static int getQueuedTaskCount() {
    Thread t;
    ForkJoinPool.WorkQueue q;
    if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
      q = ((ForkJoinWorkerThread) t).workQueue;
    else
      q = ForkJoinPool.commonSubmitterQueue();
    return (q == null) ? 0 : q.queueSize();
  }

  /**
   * Returns an estimate of how many more locally queued tasks are held by the current worker thread
   * than there are other worker threads that might steal them, or zero if this thread is not
   * operating in a ForkJoinPool. This value may be useful for heuristic decisions about whether to
   * fork other tasks. In many usages of ForkJoinTasks, at steady state, each worker should aim to
   * maintain a small constant surplus (for example, 3) of tasks, and to process computations
   * locally if this threshold is exceeded.
   *
   * @return the surplus number of tasks, which may be negative
   */
  public static int getSurplusQueuedTaskCount() {
    return ForkJoinPool.getSurplusQueuedTaskCount();
  }

  /**
   * Returns, but does not unschedule or execute, a task queued by the current thread but not yet
   * executed, if one is immediately available. There is no guarantee that this task will actually
   * be polled or executed next. Conversely, this method may return null even if a task exists but
   * cannot be accessed without contention with other threads. This method is designed primarily to
   * support extensions, and is unlikely to be useful otherwise.
   *
   * @return the next task, or {@code null} if none are available
   */
  protected static ForkJoinTask<?> peekNextLocalTask() {
    Thread t;
    ForkJoinPool.WorkQueue q;
    if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
      q = ((ForkJoinWorkerThread) t).workQueue;
    else
      q = ForkJoinPool.commonSubmitterQueue();
    return (q == null) ? null : q.peek();
  }

  /**
   * Unschedules and returns, without executing, the next task queued by the current thread but not
   * yet executed, if the current thread is operating in a ForkJoinPool. This method is designed
   * primarily to support extensions, and is unlikely to be useful otherwise.
   *
   * @return the next task, or {@code null} if none are available
   */
  protected static ForkJoinTask<?> pollNextLocalTask() {
    Thread t;
    return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
        ? ((ForkJoinWorkerThread) t).workQueue.nextLocalTask()
        : null;
  }

  // public methods

  /**
   * If the current thread is operating in a ForkJoinPool, unschedules and returns, without
   * executing, the next task queued by the current thread but not yet executed, if one is
   * available, or if not available, a task that was forked by some other thread, if available.
   * Availability may be transient, so a {@code null} result does not necessarily imply quiescence
   * of the pool this task is operating in. This method is designed primarily to support extensions,
   * and is unlikely to be useful otherwise.
   *
   * @return a task, or {@code null} if none are available
   */
  protected static ForkJoinTask<?> pollTask() {
    Thread t;
    ForkJoinWorkerThread wt;
    return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
        ? (wt = (ForkJoinWorkerThread) t).pool.nextTaskFor(wt.workQueue)
        : null;
  }

  /**
   * If the current thread is operating in a ForkJoinPool, unschedules and returns, without
   * executing, a task externally submitted to the pool, if one is available. Availability may be
   * transient, so a {@code null} result does not necessarily imply quiescence of the pool. This
   * method is designed primarily to support extensions, and is unlikely to be useful otherwise.
   *
   * @return a task, or {@code null} if none are available
   * @since 9
   */
  protected static ForkJoinTask<?> pollSubmission() {
    Thread t;
    return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
        ? ((ForkJoinWorkerThread) t).pool.pollSubmission()
        : null;
  }

  /**
   * Returns a new {@code ForkJoinTask} that performs the {@code run} method of the given
   * {@code Runnable} as its action, and returns a null result upon {@link #join}.
   *
   * @param runnable the runnable action
   * @return the task
   */
  public static ForkJoinTask<?> adapt(Runnable runnable) {
    return new AdaptedRunnableAction(runnable);
  }

  /**
   * Returns a new {@code ForkJoinTask} that performs the {@code run} method of the given
   * {@code Runnable} as its action, and returns the given result upon {@link #join}.
   *
   * @param runnable the runnable action
   * @param result the result upon completion
   * @param <T> the type of the result
   * @return the task
   */
  public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) {
    return new AdaptedRunnable<T>(runnable, result);
  }

  /**
   * Returns a new {@code ForkJoinTask} that performs the {@code call} method of the given
   * {@code Callable} as its action, and returns its result upon {@link #join}, translating any
   * checked exceptions encountered into {@code RuntimeException}.
   *
   * @param callable the callable action
   * @param <T> the type of the callable's result
   * @return the task
   */
  public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) {
    return new AdaptedCallable<T>(callable);
  }

  /**
   * Marks completion and wakes up threads waiting to join this task.
   *
   * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
   * @return completion status on exit
   */
  private int setCompletion(int completion) {
    for (int s;;) {
      if ((s = status) < 0)
        return s;
      if (U.compareAndSwapInt(this, STATUS, s, s | completion)) {
        if ((s >>> 16) != 0)
          synchronized (this) {
            notifyAll();
          }
        return completion;
      }
    }
  }

  /**
   * Primary execution method for stolen tasks. Unless done, calls exec and records status if
   * completed, but doesn't wait for completion otherwise.
   *
   * @return status on exit from this method
   */
  final int doExec() {
    int s;
    boolean completed;
    if ((s = status) >= 0) {
      try {
        completed = exec();
      } catch (Throwable rex) {
        return setExceptionalCompletion(rex);
      }
      if (completed)
        s = setCompletion(NORMAL);
    }
    return s;
  }

  /**
   * If not done, sets SIGNAL status and performs Object.wait(timeout). This task may or may not be
   * done on exit. Ignores interrupts.
   *
   * @param timeout using Object.wait conventions.
   */
  final void internalWait(long timeout) {
    int s;
    if ((s = status) >= 0 && // force completer to issue notify
        U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
      synchronized (this) {
        if (status >= 0)
          try {
            wait(timeout);
          } catch (InterruptedException ie) {
          }
        else
          notifyAll();
      }
    }
  }

  /**
   * Blocks a non-worker-thread until completion.
   *
   * @return status upon completion
   */
  private int externalAwaitDone() {
    int s = ((this instanceof CountedCompleter) ? // try helping
        ForkJoinPool.common.externalHelpComplete((CountedCompleter<?>) this, 0)
        : ForkJoinPool.common.tryExternalUnpush(this) ? doExec() : 0);
    if (s >= 0 && (s = status) >= 0) {
      boolean interrupted = false;
      do {
        if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
          synchronized (this) {
            if (status >= 0) {
              try {
                wait(0L);
              } catch (InterruptedException ie) {
                interrupted = true;
              }
            } else
              notifyAll();
          }
        }
      } while ((s = status) >= 0);
      if (interrupted)
        Thread.currentThread().interrupt();
    }
    return s;
  }

  /**
   * Blocks a non-worker-thread until completion or interruption.
   */
  private int externalInterruptibleAwaitDone() throws InterruptedException {
    int s;
    if (Thread.interrupted())
      throw new InterruptedException();
    if ((s = status) >= 0 && (s = ((this instanceof CountedCompleter)
        ? ForkJoinPool.common.externalHelpComplete((CountedCompleter<?>) this, 0)
        : ForkJoinPool.common.tryExternalUnpush(this) ? doExec() : 0)) >= 0) {
      while ((s = status) >= 0) {
        if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
          synchronized (this) {
            if (status >= 0)
              wait(0L);
            else
              notifyAll();
          }
        }
      }
    }
    return s;
  }

  /**
   * Implementation for join, get, quietlyJoin. Directly handles only cases of already-completed,
   * external wait, and unfork+exec. Others are relayed to ForkJoinPool.awaitJoin.
   *
   * @return status upon completion
   */
  private int doJoin() {
    int s;
    Thread t;
    ForkJoinWorkerThread wt;
    ForkJoinPool.WorkQueue w;
    return (s = status) < 0 ? s
        : ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
            ? (w = (wt = (ForkJoinWorkerThread) t).workQueue).tryUnpush(this) && (s = doExec()) < 0
                ? s
                : wt.pool.awaitJoin(w, this, 0L)
            : externalAwaitDone();
  }

  /**
   * Implementation for invoke, quietlyInvoke.
   *
   * @return status upon completion
   */
  private int doInvoke() {
    int s;
    Thread t;
    ForkJoinWorkerThread wt;
    return (s = doExec()) < 0 ? s
        : ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
            ? (wt = (ForkJoinWorkerThread) t).pool.awaitJoin(wt.workQueue, this, 0L)
            : externalAwaitDone();
  }

  /**
   * Records exception and sets status.
   *
   * @return status on exit
   */
  final int recordExceptionalCompletion(Throwable ex) {
    int s;
    if ((s = status) >= 0) {
      int h = System.identityHashCode(this);
      final ReentrantLock lock = exceptionTableLock;
      lock.lock();
      try {
        expungeStaleExceptions();
        ExceptionNode[] t = exceptionTable;
        int i = h & (t.length - 1);
        for (ExceptionNode e = t[i];; e = e.next) {
          if (e == null) {
            t[i] = new ExceptionNode(this, ex, t[i], exceptionTableRefQueue);
            break;
          }
          if (e.get() == this) // already present
            break;
        }
      } finally {
        lock.unlock();
      }
      s = setCompletion(EXCEPTIONAL);
    }
    return s;
  }

  /**
   * Records exception and possibly propagates.
   *
   * @return status on exit
   */
  private int setExceptionalCompletion(Throwable ex) {
    int s = recordExceptionalCompletion(ex);
    if ((s & DONE_MASK) == EXCEPTIONAL)
      internalPropagateException(ex);
    return s;
  }

  /**
   * Hook for exception propagation support for tasks with completers.
   */
  void internalPropagateException(Throwable ex) {}

  /**
   * Removes exception node and clears status.
   */
  private void clearExceptionalCompletion() {
    int h = System.identityHashCode(this);
    final ReentrantLock lock = exceptionTableLock;
    lock.lock();
    try {
      ExceptionNode[] t = exceptionTable;
      int i = h & (t.length - 1);
      ExceptionNode e = t[i];
      ExceptionNode pred = null;
      while (e != null) {
        ExceptionNode next = e.next;
        if (e.get() == this) {
          if (pred == null)
            t[i] = next;
          else
            pred.next = next;
          break;
        }
        pred = e;
        e = next;
      }
      expungeStaleExceptions();
      status = 0;
    } finally {
      lock.unlock();
    }
  }

  /**
   * Returns a rethrowable exception for this task, if available. To provide accurate stack traces,
   * if the exception was not thrown by the current thread, we try to create a new exception of the
   * same type as the one thrown, but with the recorded exception as its cause. If there is no such
   * constructor, we instead try to use a no-arg constructor, followed by initCause, to the same
   * effect. If none of these apply, or any fail due to other exceptions, we return the recorded
   * exception, which is still correct, although it may contain a misleading stack trace.
   *
   * @return the exception, or null if none
   */
  private Throwable getThrowableException() {
    int h = System.identityHashCode(this);
    ExceptionNode e;
    final ReentrantLock lock = exceptionTableLock;
    lock.lock();
    try {
      expungeStaleExceptions();
      ExceptionNode[] t = exceptionTable;
      e = t[h & (t.length - 1)];
      while (e != null && e.get() != this)
        e = e.next;
    } finally {
      lock.unlock();
    }
    Throwable ex;
    if (e == null || (ex = e.ex) == null)
      return null;
    if (e.thrower != Thread.currentThread().getId()) {
      try {
        Constructor<?> noArgCtor = null;
        // public ctors only
        for (Constructor<?> c : ex.getClass().getConstructors()) {
          Class<?>[] ps = c.getParameterTypes();
          if (ps.length == 0)
            noArgCtor = c;
          else if (ps.length == 1 && ps[0] == Throwable.class)
            return (Throwable) c.newInstance(ex);
        }
        if (noArgCtor != null) {
          Throwable wx = (Throwable) noArgCtor.newInstance();
          wx.initCause(ex);
          return wx;
        }
      } catch (Exception ignore) {
      }
    }
    return ex;
  }

  /**
   * Throws exception, if any, associated with the given status.
   */
  private void reportException(int s) {
    if (s == CANCELLED)
      throw new CancellationException();
    if (s == EXCEPTIONAL)
      rethrow(getThrowableException());
  }

  /**
   * Arranges to asynchronously execute this task in the pool the current task is running in, if
   * applicable, or using the {@link ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}.
   * While it is not necessarily enforced, it is a usage error to fork a task more than once unless
   * it has completed and been reinitialized. Subsequent modifications to the state of this task or
   * any data it operates on are not necessarily consistently observable by any thread other than
   * the one executing it unless preceded by a call to {@link #join} or related methods, or a call
   * to {@link #isDone} returning {@code
   * true}.
   *
   * @return {@code this}, to simplify usage
   */
  public final ForkJoinTask<V> fork() {
    Thread t;
    if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
      ((ForkJoinWorkerThread) t).workQueue.push(this);
    else
      ForkJoinPool.common.externalPush(this);
    return this;
  }

  /**
   * Returns the result of the computation when it {@linkplain #isDone is done}. This method differs
   * from {@link #get()} in that abnormal completion results in {@code RuntimeException} or
   * {@code Error}, not {@code ExecutionException}, and that interrupts of the calling thread do
   * <em>not</em> cause the method to abruptly return by throwing {@code InterruptedException}.
   *
   * @return the computed result
   */
  public final V join() {
    int s;
    if ((s = doJoin() & DONE_MASK) != NORMAL)
      reportException(s);
    return getRawResult();
  }

  /**
   * Commences performing this task, awaits its completion if necessary, and returns its result, or
   * throws an (unchecked) {@code RuntimeException} or {@code Error} if the underlying computation
   * did so.
   *
   * @return the computed result
   */
  public final V invoke() {
    int s;
    if ((s = doInvoke() & DONE_MASK) != NORMAL)
      reportException(s);
    return getRawResult();
  }

  /**
   * Attempts to cancel execution of this task. This attempt will fail if the task has already
   * completed or could not be cancelled for some other reason. If successful, and this task has not
   * started when {@code cancel} is called, execution of this task is suppressed. After this method
   * returns successfully, unless there is an intervening call to {@link #reinitialize}, subsequent
   * calls to {@link #isCancelled}, {@link #isDone}, and {@code cancel} will return {@code true} and
   * calls to {@link #join} and related methods will result in {@code CancellationException}.
   *
   * <p>
   * This method may be overridden in subclasses, but if so, must still ensure that these properties
   * hold. In particular, the {@code cancel} method itself must not throw exceptions.
   *
   * <p>
   * This method is designed to be invoked by <em>other</em> tasks. To terminate the current task,
   * you can just return or throw an unchecked exception from its computation method, or invoke
   * {@link #completeExceptionally(Throwable)}.
   *
   * @param mayInterruptIfRunning this value has no effect in the default implementation because
   *        interrupts are not used to control cancellation.
   *
   * @return {@code true} if this task is now cancelled
   */
  public boolean cancel(boolean mayInterruptIfRunning) {
    return (setCompletion(CANCELLED) & DONE_MASK) == CANCELLED;
  }

  public final boolean isDone() {
    return status < 0;
  }

  public final boolean isCancelled() {
    return (status & DONE_MASK) == CANCELLED;
  }

  /**
   * Returns {@code true} if this task threw an exception or was cancelled.
   *
   * @return {@code true} if this task threw an exception or was cancelled
   */
  public final boolean isCompletedAbnormally() {
    return status < NORMAL;
  }

  /**
   * Returns {@code true} if this task completed without throwing an exception and was not
   * cancelled.
   *
   * @return {@code true} if this task completed without throwing an exception and was not cancelled
   */
  public final boolean isCompletedNormally() {
    return (status & DONE_MASK) == NORMAL;
  }

  // Extension methods

  /**
   * Returns the exception thrown by the base computation, or a {@code CancellationException} if
   * cancelled, or {@code null} if none or if the method has not yet completed.
   *
   * @return the exception, or {@code null} if none
   */
  public final Throwable getException() {
    int s = status & DONE_MASK;
    return ((s >= NORMAL) ? null
        : (s == CANCELLED) ? new CancellationException() : getThrowableException());
  }

  /**
   * Completes this task abnormally, and if not already aborted or cancelled, causes it to throw the
   * given exception upon {@code join} and related operations. This method may be used to induce
   * exceptions in asynchronous tasks, or to force completion of tasks that would not otherwise
   * complete. Its use in other situations is discouraged. This method is overridable, but
   * overridden versions must invoke {@code super} implementation to maintain guarantees.
   *
   * @param ex the exception to throw. If this exception is not a {@code RuntimeException} or
   *        {@code Error}, the actual exception thrown will be a {@code RuntimeException} with cause
   *        {@code ex}.
   */
  public void completeExceptionally(Throwable ex) {
    setExceptionalCompletion(
        (ex instanceof RuntimeException) || (ex instanceof Error) ? ex : new RuntimeException(ex));
  }

  /**
   * Completes this task, and if not already aborted or cancelled, returning the given value as the
   * result of subsequent invocations of {@code join} and related operations. This method may be
   * used to provide results for asynchronous tasks, or to provide alternative handling for tasks
   * that would not otherwise complete normally. Its use in other situations is discouraged. This
   * method is overridable, but overridden versions must invoke {@code super} implementation to
   * maintain guarantees.
   *
   * @param value the result value for this task
   */
  public void complete(V value) {
    try {
      setRawResult(value);
    } catch (Throwable rex) {
      setExceptionalCompletion(rex);
      return;
    }
    setCompletion(NORMAL);
  }

  /**
   * Completes this task normally without setting a value. The most recent value established by
   * {@link #setRawResult} (or {@code
   * null} by default) will be returned as the result of subsequent invocations of {@code join} and
   * related operations.
   *
   * @since 1.8
   */
  public final void quietlyComplete() {
    setCompletion(NORMAL);
  }

  /**
   * Waits if necessary for the computation to complete, and then retrieves its result.
   *
   * @return the computed result
   * @throws CancellationException if the computation was cancelled
   * @throws ExecutionException if the computation threw an exception
   * @throws InterruptedException if the current thread is not a member of a ForkJoinPool and was
   *         interrupted while waiting
   */
  public final V get() throws InterruptedException, ExecutionException {
    int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ? doJoin()
        : externalInterruptibleAwaitDone();
    if ((s &= DONE_MASK) == CANCELLED)
      throw new CancellationException();
    if (s == EXCEPTIONAL)
      throw new ExecutionException(getThrowableException());
    return getRawResult();
  }

  /**
   * Waits if necessary for at most the given time for the computation to complete, and then
   * retrieves its result, if available.
   *
   * @param timeout the maximum time to wait
   * @param unit the time unit of the timeout argument
   * @return the computed result
   * @throws CancellationException if the computation was cancelled
   * @throws ExecutionException if the computation threw an exception
   * @throws InterruptedException if the current thread is not a member of a ForkJoinPool and was
   *         interrupted while waiting
   * @throws TimeoutException if the wait timed out
   */
  public final V get(long timeout, TimeUnit unit)
      throws InterruptedException, ExecutionException, TimeoutException {
    int s;
    long nanos = unit.toNanos(timeout);
    if (Thread.interrupted())
      throw new InterruptedException();
    if ((s = status) >= 0 && nanos > 0L) {
      long d = System.nanoTime() + nanos;
      long deadline = (d == 0L) ? 1L : d; // avoid 0
      Thread t = Thread.currentThread();
      if (t instanceof ForkJoinWorkerThread) {
        ForkJoinWorkerThread wt = (ForkJoinWorkerThread) t;
        s = wt.pool.awaitJoin(wt.workQueue, this, deadline);
      } else if ((s = ((this instanceof CountedCompleter)
          ? ForkJoinPool.common.externalHelpComplete((CountedCompleter<?>) this, 0)
          : ForkJoinPool.common.tryExternalUnpush(this) ? doExec() : 0)) >= 0) {
        long ns, ms; // measure in nanosecs, but wait in millisecs
        while ((s = status) >= 0 && (ns = deadline - System.nanoTime()) > 0L) {
          if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L
              && U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
            synchronized (this) {
              if (status >= 0)
                wait(ms); // OK to throw InterruptedException
              else
                notifyAll();
            }
          }
        }
      }
    }
    if (s >= 0)
      s = status;
    if ((s &= DONE_MASK) != NORMAL) {
      if (s == CANCELLED)
        throw new CancellationException();
      if (s != EXCEPTIONAL)
        throw new TimeoutException();
      throw new ExecutionException(getThrowableException());
    }
    return getRawResult();
  }

  /**
   * Joins this task, without returning its result or throwing its exception. This method may be
   * useful when processing collections of tasks when some have been cancelled or otherwise known to
   * have aborted.
   */
  public final void quietlyJoin() {
    doJoin();
  }

  // tag operations

  /**
   * Commences performing this task and awaits its completion if necessary, without returning its
   * result or throwing its exception.
   */
  public final void quietlyInvoke() {
    doInvoke();
  }

  /**
   * Resets the internal bookkeeping state of this task, allowing a subsequent {@code fork}. This
   * method allows repeated reuse of this task, but only if reuse occurs when this task has either
   * never been forked, or has been forked, then completed and all outstanding joins of this task
   * have also completed. Effects under any other usage conditions are not guaranteed. This method
   * may be useful when executing pre-constructed trees of subtasks in loops.
   *
   * <p>
   * Upon completion of this method, {@code isDone()} reports {@code false}, and
   * {@code getException()} reports {@code
   * null}. However, the value returned by {@code getRawResult} is unaffected. To clear this value,
   * you can invoke {@code
   * setRawResult(null)}.
   */
  public void reinitialize() {
    if ((status & DONE_MASK) == EXCEPTIONAL)
      clearExceptionalCompletion();
    else
      status = 0;
  }

  /**
   * Tries to unschedule this task for execution. This method will typically (but is not guaranteed
   * to) succeed if this task is the most recently forked task by the current thread, and has not
   * commenced executing in another thread. This method may be useful when arranging alternative
   * local processing of tasks that could have been, but were not, stolen.
   *
   * @return {@code true} if unforked
   */
  public boolean tryUnfork() {
    Thread t;
    return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
        ? ((ForkJoinWorkerThread) t).workQueue.tryUnpush(this)
        : ForkJoinPool.common.tryExternalUnpush(this));
  }

  /**
   * Returns the result that would be returned by {@link #join}, even if this task completed
   * abnormally, or {@code null} if this task is not known to have been completed. This method is
   * designed to aid debugging, as well as to support extensions. Its use in any other context is
   * discouraged.
   *
   * @return the result, or {@code null} if not completed
   */
  public abstract V getRawResult();

  /**
   * Forces the given value to be returned as a result. This method is designed to support
   * extensions, and should not in general be called otherwise.
   *
   * @param value the value
   */
  protected abstract void setRawResult(V value);

  /**
   * Immediately performs the base action of this task and returns true if, upon return from this
   * method, this task is guaranteed to have completed normally. This method may return false
   * otherwise, to indicate that this task is not necessarily complete (or is not known to be
   * complete), for example in asynchronous actions that require explicit invocations of completion
   * methods. This method may also throw an (unchecked) exception to indicate abnormal exit. This
   * method is designed to support extensions, and should not in general be called otherwise.
   *
   * @return {@code true} if this task is known to have completed normally
   */
  protected abstract boolean exec();

  /**
   * Returns the tag for this task.
   *
   * @return the tag for this task
   * @since 1.8
   */
  public final short getForkJoinTaskTag() {
    return (short) status;
  }

  /**
   * Atomically sets the tag value for this task and returns the old value.
   *
   * @param newValue the new tag value
   * @return the previous value of the tag
   * @since 1.8
   */
  public final short setForkJoinTaskTag(short newValue) {
    for (int s;;) {
      if (U.compareAndSwapInt(this, STATUS, s = status, (s & ~SMASK) | (newValue & SMASK)))
        return (short) s;
    }
  }

  /**
   * Atomically conditionally sets the tag value for this task. Among other applications, tags can
   * be used as visit markers in tasks operating on graphs, as in methods that check: {@code
   * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))} before processing, otherwise
   * exiting because the node has already been visited.
   *
   * @param expect the expected tag value
   * @param update the new tag value
   * @return {@code true} if successful; i.e., the current value was equal to {@code expect} and was
   *         changed to {@code update}.
   * @since 1.8
   */
  public final boolean compareAndSetForkJoinTaskTag(short expect, short update) {
    for (int s;;) {
      if ((short) (s = status) != expect)
        return false;
      if (U.compareAndSwapInt(this, STATUS, s, (s & ~SMASK) | (update & SMASK)))
        return true;
    }
  }

  /**
   * Saves this task to a stream (that is, serializes it).
   *
   * @param s the stream
   * @throws java.io.IOException if an I/O error occurs
   * @serialData the current run status and the exception thrown during execution, or {@code null}
   *             if none
   */
  private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
    s.defaultWriteObject();
    s.writeObject(getException());
  }

  // Serialization support

  /**
   * Reconstitutes this task from a stream (that is, deserializes it).
   *
   * @param s the stream
   * @throws ClassNotFoundException if the class of a serialized object could not be found
   * @throws java.io.IOException if an I/O error occurs
   */
  private void readObject(java.io.ObjectInputStream s)
      throws java.io.IOException, ClassNotFoundException {
    s.defaultReadObject();
    Object ex = s.readObject();
    if (ex != null)
      setExceptionalCompletion((Throwable) ex);
  }

  /**
   * Key-value nodes for exception table. The chained hash table uses identity comparisons, full
   * locking, and weak references for keys. The table has a fixed capacity because it only maintains
   * task exceptions long enough for joiners to access them, so should never become very large for
   * sustained periods. However, since we do not know when the last joiner completes, we must use
   * weak references and expunge them. We do so on each operation (hence full locking). Also, some
   * thread in any ForkJoinPool will call helpExpungeStaleExceptions when its pool becomes
   * isQuiescent.
   */
  static final class ExceptionNode extends WeakReference<ForkJoinTask<?>> {
    final Throwable ex;
    final long thrower; // use id not ref to avoid weak cycles
    final int hashCode; // store task hashCode before weak ref disappears
    ExceptionNode next;

    ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next,
        ReferenceQueue<ForkJoinTask<?>> exceptionTableRefQueue) {
      super(task, exceptionTableRefQueue);
      this.ex = ex;
      this.next = next;
      this.thrower = Thread.currentThread().getId();
      this.hashCode = System.identityHashCode(task);
    }
  }

  /**
   * Adapter for Runnables. This implements RunnableFuture to be compliant with
   * AbstractExecutorService constraints when used in ForkJoinPool.
   */
  static final class AdaptedRunnable<T> extends ForkJoinTask<T> implements RunnableFuture<T> {
    private static final long serialVersionUID = 5232453952276885070L;
    final Runnable runnable;
    T result;

    AdaptedRunnable(Runnable runnable, T result) {
      this.runnable = Objects.requireNonNull(runnable);
      this.result = result; // OK to set this even before completion
    }

    public final T getRawResult() {
      return result;
    }

    public final void setRawResult(T v) {
      result = v;
    }

    public final boolean exec() {
      runnable.run();
      return true;
    }

    public final void run() {
      invoke();
    }

    public String toString() {
      return super.toString() + "[Wrapped task = " + runnable + "]";
    }
  }

  /**
   * Adapter for Runnables without results.
   */
  static final class AdaptedRunnableAction extends ForkJoinTask<Void>
      implements RunnableFuture<Void> {
    private static final long serialVersionUID = 5232453952276885070L;
    final Runnable runnable;

    AdaptedRunnableAction(Runnable runnable) {
      this.runnable = Objects.requireNonNull(runnable);
    }

    public final Void getRawResult() {
      return null;
    }

    public final void setRawResult(Void v) {}

    public final boolean exec() {
      runnable.run();
      return true;
    }

    public final void run() {
      invoke();
    }

    public String toString() {
      return super.toString() + "[Wrapped task = " + runnable + "]";
    }
  }

  /**
   * Adapter for Runnables in which failure forces worker exception.
   */
  static final class RunnableExecuteAction extends ForkJoinTask<Void> {
    private static final long serialVersionUID = 5232453952276885070L;
    final Runnable runnable;

    RunnableExecuteAction(Runnable runnable) {
      this.runnable = Objects.requireNonNull(runnable);
    }

    public final Void getRawResult() {
      return null;
    }

    public final void setRawResult(Void v) {}

    public final boolean exec() {
      runnable.run();
      return true;
    }

    void internalPropagateException(Throwable ex) {
      rethrow(ex); // rethrow outside exec() catches.
    }
  }

  /**
   * Adapter for Callables.
   */
  static final class AdaptedCallable<T> extends ForkJoinTask<T> implements RunnableFuture<T> {
    private static final long serialVersionUID = 2838392045355241008L;
    final Callable<? extends T> callable;
    T result;

    AdaptedCallable(Callable<? extends T> callable) {
      this.callable = Objects.requireNonNull(callable);
    }

    public final T getRawResult() {
      return result;
    }

    public final void setRawResult(T v) {
      result = v;
    }

    public final boolean exec() {
      try {
        result = callable.call();
        return true;
      } catch (RuntimeException rex) {
        throw rex;
      } catch (Exception ex) {
        throw new RuntimeException(ex);
      }
    }

    public final void run() {
      invoke();
    }

    public String toString() {
      return super.toString() + "[Wrapped task = " + callable + "]";
    }
  }
}
